How to move all parts to the center, based on selected object
It sounds like you need to adjust the object pivot point? A lot of rotation and centering problems can happen if an object's pivot point is way off. In the image below, there is a gun and three separate objects. The gun is the parent, and the objects are children. You can see how the gun rotates around the pivot point, and the children follow.


Using the 3d cursor and the "Set Origin to 3D Cursor" option, you can place your object's pivot point wherever you like, including the barrel or sight. I hope this helps.